1. Field of the Invention
The present invention generally relates to protecting composite structures from lightning strikes, and more particularly, electrically connecting multiple conductive appliqués together in a continuous current path, for example, for establishing a preferred current path to ground along the areas protected by Lightning Strike Appliqué, or electrically connecting the appliqué to a ground contact for the purposes of preventing the buildup precipitation-static (p-static) charge on the aircraft surface or on underlying components.
2. Background Description
The ability to effectively manage lightning strikes on composite materials that form structural panels for wings, fuselages, fuel tanks, and other components of an aircraft structure is an important consideration for the safety of an aircraft.
Composite materials are highly desirable for use as structural components due to their lower mass, while possessing excellent structural rigidity and high strength. However, composite materials are not highly conductive and cannot dissipate the energy from a lightning strike as efficiently as traditional metal body components used in many conventional aircraft.
Carbon fiber reinforced plastic (CFRP) is one type of composite material used for skin, spar and rib installations on aircraft. A CFRP structure is about 2000 times more resistive than most metals, and consequently CFRP is more prone to electrical breakdown when subjected to currents from lightning strikes, especially at interfaces and fasteners.
Moreover, protection is needed against lightning strikes for not only composite skins and underlying structures, but for sensitive equipment, like hydraulic lines and fuel tanks, as well.
Appliqué coatings, such as Lightning Strike Appliqué (LSA), which contain a thin metal foil, and Wide Area Lightning Diverter Overlay (WALDO), are used to protect aircraft. These coatings are described in detail in US Patent Application 2006/0051592, which is incorporated herein by reference.
When using a lightning protection approach, such as LSA/WALDO, to protect the composite structure it is important to reliably transition the current that is carried by or on the appliqué coating system to a grounded metallic structural component or current return network.
Typical current return networks used on aircraft are buried inside the structure. This solution forces a designer to drive high electrical currents into the skin and composite structure itself. High currents damage sites at each electrical discontinuity, including fasteners, joints, fiber interfaces, panel edges, and the like, as well as creating hot spots, edge-glow or sparks, which, for example, could ignite the fuel within the wing box.
The difficulty of predicting where currents go once an aircraft is struck by lightning, leads to over-designing many areas of the structure and to the duplication of protection schemes.